This invention relates to a method and apparatus for focusing servo control.
It is the current practice to read the data recorded on an optical disc recording track by applying a laser beam through an objective lens which focuses a light spot on the recording track. In order to provide clear data recognition, the objective lens should be adjusted, with great accuracy, at a proper position with respect to the optical disc recording track. For this purpose, a deviation of the actual objective lens position from the optimum objective lens position is converted into a focusing error signal which is applied to a closed loop focusing servo circuit operable to reduce the focusing error signal to zero. Various methods, which includes an astigmatic method, a knife edge method, a Foucault prism method, a critical angle method and the like, have been proposed to produce such a focusing error signal. In either of these methods, a light beam reflected from the optical disc is applied through a lens which focuses a light spot on the divided portions of a photoelectric converter element. However, the reflected light beam includes 0-order and .+-. first-order diffraction beams which interfere on the photoelectric converter. Each time the light spot traverses the optical disc recording track, the intensities of the 0-order and .+-. first-order diffraction beams change greatly to superimpose a great disturbance on the focusing error signal. The disturbance has serious influence on the focusing servo control in a direction saturating the dynamic range of the focusing servo circuit to place the focusing servo control into a lock-off condition particularly during an access mode of operation searching the information recorded on the optical disc recording track.